Is there anything Kathryn Hunter can't play? In recent years she has tackled everything from Shakespeare's bewildered Lear to Lorca's barren Yerma. Now, in Colin Teevan's adaptation of Kafka's A Report to an Academy, she plays a lecturer reminiscing about his former life as an ape. The result is a performance of staggering versatility in which the human and the simian seem to be in constant contention.

As so often in Kafka, transformation makes a satiric point. In Metamorphosis Gregor's change into an insect becomes a comment on outsiderish isolation. Here the enforced evolution of the ape, Red Peter, exposes the degradation of the human animal. As the hero recalls his shipboard incarceration, we realise that his only way of escaping death was to become like his captors. So he imitates their rum-swigging coarseness, acquires the rudiments of language and ends up as a music-hall performer.

In five short years, as he mordantly says: "I reached the cultural level of an average European." But the brilliance of Hunter's performance lies in its suggestion that the hero remains tragically torn between two worlds.

In ill-fitting evening dress, Hunter stands at a lectern in swivel-eyed panic. As she re-lives Red Peter's memories of captivity, she becomes more ape-like. She squats with knuckles pressed against the ground and feeds off imaginary fleas plucked from the hair of front row spectators. Even when the rubber-limbed Hunter does vaudevillian splits, it is simply as if one form of entrapment has been exchanged for another.

Walter Meierjohann's production uses Hunter's astonishing physical plasticity to rub home Kafka's point: that humans are no more free than their animal ancestors and that both exist in a state of frenetic isolation. Hunter cuts an unforgettably solitary figure as she talks of her aversion to human beings, yet gazes at the image of a half-trained female chimpanzee with a mixture of pity and revulsion. There is much sly humour in her remarkable shape-shifting performance. But the ultimate impression, as in so many of Kafka's fables, is that the fate of modern mankind is to be out of tune with the times and hopelessly ill at ease inside his own skin.